Telephone companies offer customers a number of ways to transport data. One popular way is called Asymmetric Digital Subscriber Line (ADSL). In ADSL, a small portion of the frequency spectrum is used for communicating data from the customer to the central office, and a much larger portion of the frequency spectrum is used for communicating data from the central office to the customer. Each portion of the ADSL frequency spectrum carries data using discrete multi-tome (DMT) modulation, a technique in which data is carried on equally-spaced carrier signals. The combined number of carrier signals from both portions of the spectrum is implementation-dependent. ADSL implementations that comply with the ITU-T G.992.1 standard have 256 carrier signals, while implementations that comply with the ITU-T G.992.2 standard have 128 carrier signals. Future implementations are expected to have 512 or even 1024 carrier signals (see, e.g., ITU-T G.992.5).
DMT modulation provides for very efficient use of the available communication spectrum because the amount of data carried by each carrier signal is individually customized to fit the signal-to-noise ratio profile of the channel. Each carrier signal is allocated a number of data bits, and the allocation of bits may be dynamically adjusted as channel conditions change. Each carrier signal may also be allocated a small individual gain factor to further improve communications performance. The allocation of bits and gain factors to carrier signals is typically performed using tables.
Three tables are typically employed: a gain table, a bit table, and a tone table. The gain table specifies an individual gain factor for each carrier signal. The bit table specifies an individual number of bits allocated to each carrier signal. The tone table may be used to allocate specific data bits to specific carrier signals.
Some channels actually have two signal-to-noise ratio profiles. An example of such a channel is a twisted wire pair in a binder that also carries TCM-ISDN (Time Compression Multiplexing—Integrated Services Digital Network) traffic. TCM-ISDN employs time division multiplexing at a universal, synchronized rate of 400 Hz, i.e. the central office alternately transmits data for 1.25 milliseconds, then listens for data from the customer for 1.25 milliseconds. This causes other channels to experience a noise profile that alternates at a rate of 400 Hz.
The interference can be divided into two types: near-end cross talk (NEXT) and far-end cross talk (FEXT). NEXT results from transmissions traveling in the same direction, while FEXT results from transmissions traveling in opposite directions. Thus, central office transmissions on channels other than a given channel cause NEXT interference on the given channel for the central office and FEXT interference for the customer. Transmissions from customers on channels other than the given channel cause FEXT interference for the central office and NEXT interference for the customer. From either point of view (the central office or the customer), TCM-ISDN signaling alternately causes NEXT interference and FEXT interference. The NEXT interference is generally significantly worse than the FEXT interference, although this depends on the distance that the twisted wire pair travels alongside interfering channels.
The ITU-T G.992.1 and G.992.2 standards each address TCM-ISDN interference in their respective Annex C. Two solutions are offered: dual mode solution and FEXT-only solution. In the dual mode solution, two sets of tables (gain, bit, and tone) are used. One set of tables is used to construct symbols for transmission during periods of NEXT interference (“NEXT symbols”), and the other set of tables is used to construct symbols for transmission during periods of FEXT interference (“FEXT symbols”). Although TCM-ISDN signaling uses a 50% duty cycle, it is expected that on average, only 126/340 (about 37%) of the symbols will be free of NEXT interference, and hence constructible as FEXT symbols.
The FEXT-only solution is similar to the dual mode solution except that no symbols are constructed or sent during the periods of NEXT interference. Because only FEXT symbols are used, only one set of tables is needed. Thus the dual mode solution offers a higher data rate, albeit at some additional cost in the form of additional memory for the second set of tables. In a transceiver that supports both modes, the operator may select the FEXT-only solution to safeguard service reliability.
Annex C defines the concept of a hyperframe that specifies when NEXT symbols should be used, and specifies when FEXT symbols should be used. However, the ITU-T G.992.1 and G.992.2 standards fail to provide a reliable mechanism at the remote (customer) location for properly acquiring hyperframe symbol timing. Misalignment generally causes poor training and may lead to repeated training failures. A reliable alignment technique that is compatible with these standards would be desirable.